Digital subscriber line (DSL) is a high-speed transmission technology for data transmission via a telephone twisted pair, i.e., an unshielded twisted pair (UTP). The DSL includes an asymmetrical digital subscriber line (ADSL), a very-high-bit-rate digital subscriber line (VDSL), and a single-pair high-bit-rate digital subscriber line (SHDSL), etc.
In the various digital subscriber line (xDSL) technologies, in addition to the DSLs based on baseband transmission such as Integrated Services Digital Network DSL (IDSL) and SHDSL, the DSL based on passband transmission makes the DSL and plain old telephone service (POTS) coexist on the same twisted pair using frequency division multiplexing technology. The DSL occupies a high frequency band, the POTS occupies a baseband below 4 KHz. The POTS signal and the DSL signal are split or combined through a splitter. The xDSL based on passband transmission conducts modulation and demodulation using the discrete multi-tone (DMT) modulation technology. The DMT modulation is another name of the orthogonal frequency division multiplexing (OFDM) modulation at the DSL domain. A system for providing multiple DSL access is referred to as a DSL access multiplexer (DSLAM), and a system connection relation thereof is shown in FIG. 1.
A subscriber end xDSL transceiver, i.e., a subscriber end DSLAM 120, includes a subscriber end transceiver unit 121 and a splitter 122. In an uplink direction, the subscriber end transceiver unit 121 receives a DSL signal from a computer 110, amplifies the received signal, and sends the amplified DSL signal to the splitter 122. The splitter 122 combines the DSL signal from the subscriber end transceiver unit 121 and a POTS signal from a telephone terminal 130. The combined signal is transmitted through a multiplexing UTP 140, and received by a splitter 151 in an office end xDSL transceiver 150, i.e., a DSLAM 150. The splitter 151 splits the received signal, sends the POTS signal to a public switched telephone network (PSTN) 160, and sends the DSL signal to an office end transceiver unit 152 of the office end xDSL transceiver 150. The office end transceiver unit 152 subsequently amplifies the received DSL signal and sends it to a network management system (NMS) 170. In a downlink direction, the signal is transmitted in an order reverse to the above.
Currently, the DSL technology, especially the VDSL2 technology, is widely applied to the transmission of triple play services, such as the Internet protocol television (IPTV) services. The triple play services require a lower packet loss ratio than the conventional data services. The DSL device employs noise margin and trellis coding techniques to overcome the negative impact of random noises on the transmission of packets that carry data. When the DSL runs in a twisted-pair environment with multi-pulse noises, i.e., runs in the system as shown in FIG. 1, the forward error correction (FEC) coding and interleaving techniques are adopted to provide protection against impulse noises for the transmitted data. That is, the data is obtained from the received packet by FEC coding and interleaving, and decoding, this method is called impulse noise protection (INP). However, with the development of the DSL technology, the transmission rate of the packet becomes increasingly higher, so that in order to protect a channel that bears packets at a high transmission rate against impulse noises, the interleaving memory required at the DSL device is increased. Meanwhile, as the triple play services have a higher requirement on the packet loss ratio, more interleaving memory at the DSL device is needed. The increase of the interleaving memory at the DSL device not only raises the cost of the DSL device, but also makes it difficult for the DSL device to bear services requiring a small delay such as Voice over IP (VOIP) services at the same time.
Therefore, in order to reduce the negative impact of the impulse noises so as to implement a drop in the bit error rate or packet loss ratio, when the DSL device runs in a multi-pulse noise environment, a retransmission technique may be introduced. The most famous retransmission technique is a packet retransmission technique based on transmission control protocol/Internet protocol (TCP/IP), and the basic process thereof is described as follows.
First, a sending end encapsulates data into a TCP packet and assigns a packet ID for the TCP packet, performs a cyclic redundancy check (CRC) computing on the TCP packet and attaches a computed CRC value at a particular position of the TCP packet, then sends the TCP packet to a receiving end, and meanwhile saves the TCP packet in the cache for potential retransmission.
Afterward, the receiving end receives the TCP packet and caches it, then performs a CRC on the TCP packet, and compares an obtained CRC value with the CRC value carried in the TCP packet. If it is found that the packet does not have an error, the receiving end receives the TCP packet. If it is found that the packet has an error, the receiving end sends a retransmission request carrying the packet ID to the sending end and also discards the TCP packet.
On receiving the retransmission request, the sending end acquires from the cache the TCP packet corresponding to the packet ID carried in the request, and sends the TCP packet to the receiving end again.
Finally, the receiving end correctly receives the retransmitted TCP packet without error.
The basic principle of retransmission is merely illustrated above, and other contents are recorded in the TCP/IP. Such contents only relate to how to improve the retransmission efficiency, so the details will not be given herein again.
The following defects exist in introducing the above retransmission mechanism at the DSL device running in a multi-pulse noise environment. When the packet ID is damaged, it is impossible to point out which packets need to be retransmitted, so that the retransmission mechanism may only be used in circumstances with a few packet transmission errors. In the circumstances with a few packet transmission errors, the packet ID only takes up a very small part of the packet, and the probability that the packet ID goes wrong is small. However, in circumstances with serious packet transmission errors, i.e., the content of the whole packet is almost damaged, the probability that the packet ID is damaged is high, and in such circumstances, the retransmission mechanism faces a severe problem. When the DSL device transmits data in a multi-pulse noise environment, one impulse noise usually damages the packet within a DMT symbol that is transmitted by the DSL device, and one impulse noise usually damages several DMT symbols transmitted consecutively. As the data in the packet carried by one DMT symbol amounts to thousands of bits, the reliability of initiating retransmission through the packet ID in such environment is poorer.
In addition, the above retransmission mechanism may be introduced at the DSL device running in the multi-pulse noise environment by the following two means. One is to configure the retransmission mechanism in a layer below the FEC coding layer and the interleaving layer at the DSL device. The other is to configure the retransmission mechanism in a layer above the FEC coding layer and the interleaving layer at the DSL device. The two means both have disadvantages. For the first means, if the above retransmission mechanism is configured in a layer below the FEC coding layer and the interleaving layer at the DSL device, as long as an error occurs to the transmitted packet, the error can be found through CRC and a retransmission is initiated. At this time, the packets received on the FEC coding layer and the interleaving layer at the DSL device nearly have no error, and thus the capacities of FEC coding layer and the interleaving layer are not brought into play. For the second means, if the retransmission mechanism is configured in a layer above the FEC coding layer and the interleaving layer at the DSL device, when a packet is transmitted to the FEC coding layer and the interleaving layer, errors in the packet are scattered to raise the error correction rate. If the errors in the packet exceed the error correction capability, due to error interleaving and scattering, the retransmission layer initiates more packet retransmissions, and thus more packets are retransmitted. Meanwhile, due to the interleaving delay, the retransmission request initiated by the retransmission layer has a great delay, so that the retransmission layers at the sending end and the receiving end require an even larger cache. Particularly, in order to resist large impulse noises at the DSL device, a great interleaving delay may occur, and the interleaving memory reaches 64 Kb/128 Kb. When the impulse noises are difficult to resist, the error packets resulted from the impulse noises will be spread to a large scope, so that more error packets need to be retransmitted, and the retransmission efficiency decreases.
In view of the above, when the retransmission mechanism is introduced at the DSL device running in the multi-pulse noise environment, if the interleaving technique is not used and only the FEC coding technique is adopted, the error correction capability of the FEC coding layer will be greatly reduced. Meanwhile, when the retransmission mechanism is introduced at the DSL device running in the multi-pulse noise environment, if neither the FEC coding technique nor the interleaving technique is adopted and only the retransmission mechanism is employed to ensure the correctness of the received packet, the receiving efficiency of the whole system will be decreased as the FEC coding technique is capable of not only performing error correction but also providing a 3 db coding gain.